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Abstract

Influenza causes substantial morbidity and mortality in some influenza sea-
sons, especially among the elderly. Influenza seasons dominated by circula-
tion of influenza A/H3N2 virus tend to result in more morbidity and mor-
tality than seasons dominated by influenza A/H1N1 or influenza B viruses.
Influenza viruses undergo constant mutation, called antigenic drift, which
is largely driven by host immunity. It has been shown that antigenic drift
in influenza A/H3N2 virus proceeds in a punctuated, as opposed to contin-
uous, fashion. A cluster of antigenically similar influenza A/H3N2 viruses
appears to remain dominant for between 1 and 8 influenza seasons before
being supplanted by a new cluster. Influenza seasons when a new cluster
becomes dominant may result in higher morbidity and mortality than other
seasons. Influenza vaccine effectiveness varies between influenza seasons be-
cause of the different subtypes in circulation and the degree of antigenic
match between vaccine and circulating variants. In each influenza season in
recent years, over 70% of the population of England & Wales aged > 65 has
been vaccinated, though the impact of this high coverage on population level
morbidity and mortality is unknown. Multivariate time series models were
fitted to reports of laboratory confirmed influenza, sentinel general practi-
tioner (GP) consultations for influenza-like-illness, and all deaths registered
to underlying pneumonia or influenza in England & Wales from 1975/76 to
2004/05. The models successfully distinguish influenza
- attributable GP
consultations and deaths from GP consultations and deaths that would be
expected in the absence of influenza. This distinction is made jointly by
the laboratory reports and the non-laboratory confirmed surveillance data.
It is not possible to use the multivariate time series models to quantify
the average effect of the appearance of a new cluster of influenza A/H3N2
virus variants, or vaccine impact, on influenza
- attributable morbidity or
mortality in the data analyzed. Reasons for this are discus